Recuperative Heat Exchanger, Fuel Cell System Including Recuperative Heat Exchanger, and Method of Operating Same

ABSTRACT

A heat exchanger, such as a cathode recuperator for a high temperature fuel cell system, has a corrugated separator, a barrier and a plurality of flow channels. The corrugated separator has a surface positioned along a heat exchange fluid flow path, opposite ends of the separator having flattened corrugations. The barrier is positioned adjacent the surface. The plurality of flow channels are in the heat exchange fluid flow path and are at least partially defined by the surface and the barrier. The flattened corrugations are positioned adjacent crests in the corrugated separator and secured to the barrier.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 61/095,818, filed Sep. 10, 2008, the entirecontents of which is hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to heat exchangers in general and in moreparticular applications, to recuperative heat exchangers which find manyuses in industry, including in fuel cell systems.

BACKGROUND

A recuperative heat exchanger, or recuperator, is used to optimize theoverall system efficiency of a high temperature application, such as agas turbine or a high temperature fuel cell system, by heating a lowtemperature incoming air stream to a temperature closer to the desiredprocess operating temperature via the transfer of thermal energy from ahigh temperature process waste stream of exhaust gas or air. Such a heatexchanger allows for the efficient transfer of heat from the hot streamto the cold stream while maintaining isolation of the two streams fromeach other. In order to simplify the packaging of the recuperator intothe system, and in order to reduce the material costs of the device, itis usually desirable to minimize the physical size and weight of therecuperative heat exchanger. It is also typically a principal object ofsuch a heat exchanger to provide for high heat exchanger effectivenessin order to maximize the degree to which the heat is recuperated.

Heat exchanger effectiveness is defined as the ratio between the actualrate at which heat is transferred between the two fluids in a heatexchanger and the maximum possible heat transfer rate. The maximumpossible heat transfer rate is achieved when the exit temperature of thefluid with the lower heat capacity is made to be equal to the enteringtemperature of the other fluid, and can theoretically be achieved in aheat exchanger of infinite length with the fluids passing through it ina counter-flow orientation. For most practical heat exchangers theeffectiveness will be less than one.

A cathode recuperator for high temperature fuel cell systems such as,for example, solid oxide fuel cell (SOFC) systems, has some uniqueperformance requirements as compared to recuperators in better-knownapplications such as, for example, gas turbines. SOFC's are solid-statedevices that use an oxide-conducting ceramic electrolyte to produceelectrical current by transferring oxygen ions from an oxidizing gasstream at the cathode of the fuel cell to a reducing gas stream at theanode of the fuel cell. This type of fuel cell is seen as especiallypromising in the area of distributed stationary power generation. SOFC'srequire an operating temperature range which is the highest of any fuelcell technology, giving it several advantages over other types of fuelcells for these types of applications. The rate at which a fuel cell'selectrochemical reactions proceed increases with increasing temperature,resulting in lower activation voltage losses for the SOFC. The SOFC'shigh operating temperature precludes the need for precious metalcatalysts, resulting in substantial material cost reductions. Theelevated exit temperature of the flow streams allow for high overallsystem efficiencies in combined heat and power applications, which arewell suited to distributed stationary power generation.

The traditional method of constructing solid oxide fuel cells has beenas a large bundle of individual tubular fuel cells. Systems of severalhundred kilowatts of power have been successfully constructed using thismethodology. However, there are several known disadvantages to thetubular design which severely limit the practicality of its use in thearea of 25 kW-100 kW distributed stationary power generation. Forexample, producing the tubes can require expensive fabrication methods,resulting in achievable costs per kW that are not competitive withcurrently available alternatives. As another example, the electricalinterconnects between tubes can suffer from large ohmic losses,resulting in low volumetric power densities. These disadvantages to thetubular designs have led to the development of planar SOFC designs. Theplanar designs have been demonstrated to be capable of high volumetricpower densities, and their capability of being mass produced usinginexpensive fabrication techniques is promising.

As is known in the art, a single planar solid oxide fuel cell (SOFC)consists of a solid electrolyte that has high oxygen ion conductivity,such as yttria stabilized zirconia (YSZ); a cathode material such asstrontium-doped lanthanum manganite on one side of the electrolyte,which is in contact with an oxidizing flow stream such as air; an anodematerial such as a cermet of nickel and YSZ on the opposing side of theelectrolyte, which is in contact with a fuel flow stream containinghydrogen, carbon monoxide, a gaseous hydrocarbon, or a combinationthereof such as a reformed hydrocarbon fuel; and an electricallyconductive interconnect material on the other sides of the anode andcathode. A number of these cells are assembled into a fuel cell stack,with the electrically conductive interconnect material providing boththe electrical connection between adjacent cells and the flow paths forthe reactant flow streams to contact the anode and cathode. Such cellscan be produced by well-established production methodologies such asscreen-printing and ceramic tape casting.

It is critical in operation to prevent the anode flow from mixing withthe cathode flow, since the cathode flow will act as an oxidizer tocombust the fuel in the anode flow, leading to potentially damagingcombustion occurring within the fuel cell system. High temperaturegas-tight seals are therefore required between the individual fuel cellsand the interconnect material in order to prevent such mixing fromoccurring. In order to meet the requirements of operating at hightemperatures, remaining stable in both oxidizing and reducingenvironments, and other considerations necessary for usage with SOFCs,these seals are typically constructed of cements, glasses, orglass-ceramics.

As is known to those in the art, these types of sealing materials arenot capable of withstanding large differential pressures. As aconsequence, planar SOFC systems are typically not capable of operationat elevated pressures, as are gas turbines. This has resulted in theneed for very low pressure drop, high thermal efficiency recuperativeheat exchangers to recover the waste heat from the cathode exhaust inorder to preheat the cathode air feed. The power required to pressurizethe cathode air is quite often the largest single parasitic power drawof a SOFC system, so minimizing the pressure drop in such a recuperatorcan provide substantial gains in the overall electrical efficiency ofthe system, thus potentially providing a critical commercial advantage.

SUMMARY

In some embodiments, the invention provides a primary surface annularheat exchanger suitable for use as a recuperator in solid oxide fuelcell systems.

In some embodiments, the invention provides a heat exchanger having acorrugated separator, a barrier and a plurality of flow channels. Thecorrugated separator has a surface positioned along a heat exchangefluid flow path, opposite ends of the separator having flattenedcorrugations. The barrier is positioned adjacent the surface. Theplurality of flow channels are in the heat exchange fluid flow path andare at least partially defined by the surface and the barrier. Theflattened corrugations are secured to the barrier.

The invention also provides a method of making a heat exchanger. Themethod includes the acts of providing a corrugated separator sheethaving corrugations extending in a longitudinal direction, flatteningthe corrugations into flattened portions positioned at first and secondlongitudinal ends of the corrugated separator sheet, positioning thecorrugated separator sheet adjacent a non-corrugated barrier to create aheat exchange flow path between the corrugated separator sheet and thenon-corrugated barrier, and securing the flattened portions to a surfaceof the non-corrugated barrier.

The invention provides a corrugated separator sheet for a heatexchanger. The corrugated separator sheet can include a plurality ofcorrugations and a flattened region. The plurality of corrugationsextend parallel to one another in a longitudinal direction, and have aplurality of peaks and a plurality of troughs opposite the plurality ofpeaks. The flattened region is proximate a longitudinal end of theseparator sheet and is adjacent the plurality of peaks.

The invention can also provide a primary surface annular heat exchangerthat is capable of achieving a high degree of heat exchangereffectiveness with minimal pressure drop and minimal size and weightimpact on a system making use of such a heat exchanger.

In some embodiments, the invention provides a method of constructing aprimary surface annular heat exchanger to exchange heat between twoflowstreams, the method providing reliable sealing of the flowstreamsfrom one another with a minimum number of parts and low overall cost.

In one aspect of the invention, a primary surface annular heat exchangercomprises a corrugated separator sheet with a first surface exposed to afirst heat exchanging fluid and a second surface exposed to a secondheat exchanging fluid. The first fluid flows through a plurality of flowchannels bounded by the first surface of the corrugated separator sheetand a radially inwardly located cylinder. The second fluid flows througha plurality of flow channels bounded by the second surface of thecorrugated separator sheet and a radially outwardly located cylinder.Each of the ends of the corrugated separator sheet has the corrugationsflattened and bonded to the radially inwardly located cylinder.

In another aspect of the invention, a method is provided forconstructing a primary surface annular heat exchanger. The method ofmaking the heat exchanger includes the steps of corrugating a separatorsheet and forming it into a corrugated cylinder by joining a firstcorrugation located at a first edge oriented parallel to thecorrugations and a second corrugation located at a second edge orientedparallel to the corrugations. The method of making the heat exchangermay further include the steps of flattening the corrugations at eitherend of the corrugated cylinder, and bonding the flattened portions ofthe corrugations to the surface of a non-corrugated cylinder.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan section view of a portion of a fuel cell systememploying a heat exchanger according to an embodiment of the presentinvention;

FIG. 2 is a detail view of the portion II-II of the heat exchanger ofFIG. 1;

FIG. 3 is a perspective view of a single convolution of a primarysurface of a heat exchanger according to an embodiment of the presentinvention;

FIG. 4 is an elevation view of a corrugated separator for use in a heatexchanger according to an embodiment of the present invention;

FIG. 5 is a partial perspective view of portions of a heat exchangeraccording to an embodiment of the present invention;

FIG. 6 is a somewhat diagrammatic section view through a flow channel ofa heat exchanger according to the present invention in order toillustrate the flowpaths;

FIG. 7 is a somewhat diagrammatic section view similar to a portion ofFIG. 6 but showing aspects of an alternate embodiment of a heatexchanger according to the present invention;

FIG. 8 is a section view in the direction VIII-VIII of FIG. 6;

FIG. 9 is a perspective view of a corrugated separator for use in a heatexchanger at a stage of manufacture according to an embodiment of thepresent invention;

FIG. 10 is a detail view of the portion X-X of FIG. 9;

FIG. 11 is a somewhat diagrammatic view of a stage of manufacture of aheat exchanger according to an embodiment of the present invention; and

FIG. 12 is a diagrammatic view illustrating the geometric relationshipbetween certain of the components shown in FIG. 11.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. Also, it is to be understood thatthe phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Unless specified or limited otherwise, theterms “mounted,” “connected,” “supported,” and “coupled” and variationsthereof are used broadly and encompass both direct and indirectmountings, connections, supports, and couplings. Further, “connected”and “coupled” are not restricted to physical or mechanical connectionsor couplings.

FIG. 1 illustrates an embodiment of a heat exchanger according to thepresent invention employed in a solid oxide fuel cell system. As hasbeen previously disclosed in pending U.S. patent application2008/0038622 to Valensa et. al., whose contents are hereby incorporatedby reference in their entirety, a recuperative heat exchanger to preheatcathode air for solid oxide fuel cells using a cathode exhaust can beincorporated in an annular volume surrounding the solid oxide fuel cellstacks and associated high-temperature balance-of-plant in a solid oxidefuel system. As shown in FIG. 1, an embodiment of a primary surfacerecuperative heat exchanger 1 according to the present invention is soincorporated into a solid oxide fuel cell system 100. The heat exchanger1 is arranged in an annular volume surrounding the solid oxide fuel cellstacks and associated high-temperature balance-of-plant, collectively101. The advantages of such an arrangement has been well-discussed inthe aforementioned application 2008/0038622.

As can be better seen in FIG. 2, the heat exchanger 1 as depicted in theembodiment of FIG. 1 comprises a first plurality of flowpaths A and asecond plurality of flowpaths B. The plurality of flowpaths A arebounded by a first surface 5 of a corrugated separator sheet 2 and afirst cylinder 4, or first barrier. The plurality of flowpaths B arebounded by a second surface 6 of the corrugated separator sheet 2 and asecond cylinder 3, or second barrier, said cylinder 3 being concentricto the cylinder 4 and furthermore being larger in diameter than cylinder4. The corrugated separator sheet 2 is located in the annular gapbetween the cylinders 3 and 4. In other embodiments, the first andsecond cylinders may be substantially planar barriers or barriers havingother geometrical configurations such that flowpaths A and B areconfigured to exchange heat.

A single convolution 7, or corrugation, of the corrugated separatorsheet 2 is shown in greater detail in FIG. 3. In the embodiment shown,the convolution comprises a first crest 8, or peak, formed so that thefirst separator sheet surface 5 assumes a concave shape at the crest 8and the second separator sheet surface 6 assumes a convex shape at thecrest 8, and a second crest 9, or trough, formed so that the firstseparator sheet surface 5 assumes a convex shape at the crest 9 and thesecond separator sheet surface 6 assumes a concave shape at the crest 9.A plurality of generally straight sections 22 of the corrugatedseparator sheet 2 join the crests 8 and 9 of adjacent convolutions 7.

In some embodiments, the crests 8 are joined to the cylinder 3 by amethod such as brazing, welding, gluing, or other methods of joiningknown to those skilled in the art. In some embodiments, the crests 9 arejoined to the cylinder 4 by a method such as brazing, welding, gluing,or other methods of joining known to those skilled in the art. In someembodiments, it may be preferable to have a bond between the crests 8and the cylinder 3 in only certain limited areas. In some embodiments,it may be preferable to have a bond between the crests 9 and thecylinder 4 in only certain limited areas.

Methods of corrugating a sheet to take such a form are well-known tothose skilled in the art of heat exchangers. It should be understoodthat the shape of the corrugations shown in the figures is meant to beillustrative of the overall concept, and is not meant to be limitingwith regard to the specific shape of the corrugations. In otherembodiments, the corrugated separator sheet may have other geometricalconfigurations, such as triangular corrugations (i.e., straight sectionsjoining at crests of sharp points), rectangular corrugations (i.e.,straight sections joining at flat crests) or curved corrugations (i.e.,a sinusoidal pattern), amongst others. Other types of corrugationscommonly used in heat exchangers, such as for example a corrugatedseparator sheet having flat-crested convolutions, would be equally validsubstitutes for the geometry shown.

As is best seen in the elevation view of FIG. 4, the corrugatedseparator sheet 2 can be divided into several distinct regions along theflow length of the channels A and B: a zone D1 at a first longitudinalend of the corrugated separator 2, wherein the convolutions 7 areflattened in order to seal off a first end of the plurality of flowchannels A; a zone D2 at a second longitudinal end of the corrugatedseparator 2 opposite the first longitudinal end, wherein theconvolutions 7 are similarly flattened in order to seal off a second endof the plurality of flow channels A, the second end of the flow channelsA being located opposite the first end of the flow channels A; a centerzone C wherein the convolutions are in an unflattened shape as describedabove in reference to FIG. 3; a transition zone E1 located between andconnecting the zones D1 and C; and a transition zone E2 located betweenand connecting the zones D2 and C.

The flow paths through an embodiment of the heat exchanger 1 can be seenin FIG. 6. A first fluid flow 18 enters the flow channels A by passingfirst through an annular flow channel 11 formed by the inner surface ofcylinder 4 and a wall 16 located radially inward therefrom, and secondthrough an inlet 10 in the cylinder 4. In the embodiment of FIG. 6 theinlet 10 is located in the zone C of the corrugated separator sheet 2,adjacent the zone E1. In one embodiment of the invention, depicted inFIG. 5, the inlet 10 comprises a plurality of trapezoid-shaped openingsin the cylinder 4, each opening having a 180° rotated orientation ascompared to its neighboring openings so that adjacent openings areseparated by a thin web oriented in a direction non-parallel to thedirection of the flow channels, thereby ensuring that all of the flowchannels A are fluidly connected to the annular flow channel 11.

Referring again to the embodiment of the heat exchanger 1 shown in FIG.6, the fluid flow 18 exits the flow channels A by passing through anoutlet 20 in the cylinder 4 and into an annular flow channel 12 formedby the inner surface of cylinder 4 and a wall 17 located radially inwardtherefrom, the outlet 20 being located in the zone C of the corrugatedseparator sheet 2, adjacent the zone E2. In one embodiment, the outlet20 is constructed to be similar to the inlet 10 of the embodiment shownin FIG. 5. A second fluid flow 19 passes through the flow channels B ina direction counter to the flow direction of the first fluid flow 18passing through the flow channels A. As shown in FIG. 6, the secondfluid flow 19 passes through an annular flow channel 13 located betweenthe cylinders 3 and 4, enters the flow channels B in the transition zoneE2, and exits from the flow channels B back into the annular flowchannel 13 in the transition zone E1.

An alternative embodiment, illustrated in FIG. 7, includes a radiallyexpanded section 21 of the cylinder 3. In this embodiment, the fluidflow 19 exits the flow channels B in the zone C of the corrugatedseparator sheet 2 rather than in the zone E1, and flows into an annularflow channel 14 formed by the expanded section 21 and the cylinder 4. Itshould be understood that the alternative embodiment shown in FIG. 7 canbe applied in a similar manner to the inlet end of the flow channels B.

In one embodiment, the shape of the convolutions in the zones D1 and D2is as shown in the section view of FIG. 8. The convolutions in thesezones have been folded over into a flattened portion 32 (i.e.,corresponding to D1) and flattened into the cylinder 4, thereby sealingoff the ends of the flow channels A. Regions E1 and D1, and E2 and D2,include convolutions bent in a direction transverse to the longitudinaldirection in which the convolutions extend, the convolutions overlappingadjacent convolutions in the first and second flattened regions D1 andD2.

The flattened portion 32 is positioned adjacent a crest of thecorrugated separator sheet 2. As can be seen, with reference to FIG. 5,positioning the flattened portion 32 adjacent one set of crests allowsthe flattened portion 32 to be secured to the cylinder 4 such that theset of crests do not interfere. In preferred embodiments, the flattenedportion 32 is substantially flush with the set of crests such that theflattened portion 32 and set of crests engage the cylinder 4 together.With reference to FIGS. 4-5, a second flattened portion (i.e.,corresponding to D2) is also positioned adjacent and flush with the setof crests and secured to the first cylinder 4 in the same way as thefirst flattened portion 32. In other embodiments, the second flattenedportion may be positioned adjacent and/or flush with the opposite set ofcrests and secured to second cylinder 3, or another barrier.

A process for forming a primary surface annular heat exchanger accordingto the embodiments shown in FIGS. 4, 5 and 8 will now be described withreference to FIGS. 9-12. As best seen in FIGS. 9 and 10, a corrugatedseparator sheet 2 is formed into a closed loop by engaging a convolution9 a located at a first longitudinal edge of the corrugated separatorsheet into a convolution 9 b located at a second longitudinal edge ofthe corrugated separator. In some preferable embodiments, the engagementbetween the convolutions 9 a and 9 b is secured by creating ametallurgical bond, such as by autogenous welding.

In some embodiments of the invention, the corrugated separarator sheetis next slid over a cylinder 24, the cylinder 24 having an outerdiameter that is approximately equal to the outer diameter of the innercylinder 4 of the primary surface annular heat exchanger. In aprefereable embodiment the width of the corrugated separator sheet isselected such that after engagement of the convolutions 9 a and 9 b, thecorrugated separator sheet will not fit over the cylinder 24 in a freestate. Since the nature of the convolutions allow for relatively easyexpansion of the corrugated separator sheet, a most preferableembodiment would size the width of the corrugated separator sheet sothat a slight stretching of the convolutions of the corrugated separatorsheet occurs as it is placed over the cylinder 24, thereby ensuringuniform contact between the plurality of crests 9 and the cylinder 24.

In some embodiments of the invention, the corrugations in the zones D1and D2 of the corrugated separator sheet 2 are flattened in a processillustrated in FIGS. 11 and 12. In such a process, a first wheel 22 of adiameter substantially smaller than the diameter of the cylinder 24 ispositioned to be tangent to and in contact with the inner surface of thecylinder 24, so that the axis 26 of the wheel 22 and the axis 25 of thecylinder 24 are parallel to one another and define a plane 28, the plane28 being the plane common to both axes 25 and 26. A second wheel 23 ispositioned such that the axis 27 of the second wheel 23 is parallel tothe axes 25 and 26 of the first wheel 22 and the cylinder 24, and islocated in the plane 28. The second wheel 23 is furthermore positionedso that the tangent distance H1 between the wheels 22 and 23 is lessthan the sum of the convolution height H2 of the corrugated separator 2and the thickness T1 of the cylinder 24. The wheels 22 and 23 are madeto rotate about their respective axes 26 and 27, the wheel 22 rotatingin a first direction indicated by the arrow 29 and the wheel 23 rotatingin a second direction indicated by the arrow 30, said directions beingopposite of one another, thereby causing the cylinder 24 to rotate aboutits own axis 25 in a direction indicated by the arrow 31. The rotationof the cylinder 24 causes successive convolutions of the corrugatedseparator to be compressed as they pass under the wheel 23, therebyforming the flattened zone D1 or D2 of the corrugated separator, theamount of compression being determined by the tangential spacing H1. Thewidth of the wheel 23 and the axial location of the corrugated separatorrelative to the wheel 23 can be selected in order to control the widthof the flattened zone D1 or D2 of the corrugated separator.

In some embodiments, it may be preferable for the cylinder 24 to makemultiple complete revolutions about its axis 25 and to successivelydecrease the spacing H1 while the cylinder 24 is revolving in order toflatten the convolutions in a more controlled manner.

In some embodiments of the invention multiple pairs of the wheels 22 and23 are used to flatten the convolutions at both ends of the corrugatedseparator 2 in the same operation.

In some embodiments of the invention the corrugated separator is removedfrom the cylinder 24 and is assembled over the cylinder 4. In some otherembodiments the cylinder 24 may actually be the cylinder 4.

In some embodiments of the invention the flattened zones D1 and D2 arebonded to the cylinder 4 by welding, brazing, gluing, or other bondingprocesses known to those skilled in the art.

It should be understood that the embodiments described above andillustrated in the figures are presented by way of example only and arenot intended as a limitation upon the concepts and principles of thepresent invention. As such, it will be appreciated by one havingordinary skill in the art that various changes are possible.

1. A heat exchanger comprising: a corrugated metal separator having asurface positioned along a heat exchange fluid flow path, opposite endsof the separator having flattened corrugations; a barrier positionedadjacent the surface; and a plurality of flow channels in the heatexchange fluid flow path at least partially defined by the surface andthe barrier; wherein the flattened corrugations are secured to thebarrier.
 2. The heat exchanger of claim 1, wherein the heat exchangefluid flow path is a first heat exchange fluid flow path, wherein thesurface is a first surface, wherein the barrier is a first barrier andwherein the plurality of flow channels are a first plurality of flowchannels, wherein the corrugated separator further includes a secondsurface positioned along a second heat exchange fluid flow path, theheat exchanger further comprising: a second barrier positioned adjacentthe second surface; and a second plurality of flow channels in thesecond heat exchange fluid flow path bounded by the second surface andthe second barrier; wherein the first barrier and the second barrier aresubstantially concentric cylinders, and wherein the first barrier islocated radially inward from the second barrier.
 3. The heat exchangerof claim 2, wherein the corrugated separator includes a first flattenedregion adjacent a first end of the separator, a second flattened regionadjacent a second end of the separator, a corrugated region between thefirst flattened region and the second flattened region, a firsttransition region connecting the first flattened region and a first endof the corrugated region, and a second transition region connecting thesecond flattened region and a second end of the corrugated region, andwherein the first barrier includes an inlet to the first heat exchangefluid flow path, the inlet being positioned proximate the first end ofthe corrugated region.
 4. The heat exchanger of claim 3, wherein theinlet includes a plurality of trapezoid-shaped openings.
 5. The heatexchanger of claim 3, wherein the first barrier includes an outlet fromthe first heat exchange fluid flow path, the outlet being positionedproximate the second end of the corrugated region.
 6. The heat exchangerof claim 5, wherein the outlet includes a plurality of trapezoid-shapedopenings.
 7. The heat exchanger of claim 1, wherein the corrugatedseparator includes crests, wherein the crests are secured to the barrierby one of soldering, brazing, welding, adhesive bonding, and cohesivebonding.
 8. The heat exchanger of claim 1, wherein the corrugatedseparator is formed into a substantially cylindrical shape.
 9. The heatexchanger of claim 8, wherein the corrugated separator includescorrugations extending in a direction substantially parallel to acentral axis of the substantially cylindrical shape.
 10. The heatexchanger of claim 1, wherein the corrugated separator includes alongitudinal direction in which the corrugations extend, wherein thecorrugated separator includes a first flattened region adjacent a firstlongitudinal end of the separator, a second flattened region adjacent asecond longitudinal end of the separator, a corrugated region betweenthe first flattened region and the second flattened region, a firsttransition region connecting the first flattened region and a first endof the corrugated region, and a second transition region connecting thesecond flattened region and a second end of the corrugated region, andwherein the first transition region and the first flattened regioninclude corrugations bent in a direction transverse to the longitudinaldirection, the corrugations overlapping adjacent corrugations in thefirst flattened region.
 11. The heat exchanger of claim 1, wherein theflattened corrugations secured to the barrier substantially seal off theheat exchange fluid flow path.
 12. The heat exchanger of claim 1,wherein the corrugations include a plurality of peaks and troughs, andwherein the flattened corrugations are located adjacent one of the peaksand the troughs.
 13. A method of making a heat exchanger, the methodcomprising the acts of: providing a corrugated metal separator sheethaving metal corrugations extending in a longitudinal direction;flattening the metal corrugations into flattened portions positioned atfirst and second longitudinal ends of the corrugated separator sheet;positioning the corrugated separator sheet adjacent a non-corrugatedbarrier to create a heat exchange flow path between the corrugatedseparator sheet and the non-corrugated barrier; and securing theflattened portions to a surface of the non-corrugated barrier.
 14. Themethod of claim 13, further comprising: forming the corrugated separatorsheet into a corrugated cylinder by joining a first corrugation locatedat a first edge oriented parallel to the corrugations and a secondcorrugation located at a second edge oriented parallel to thecorrugations.
 15. The method of claim 13, wherein the act of flatteningfurther comprises: positioning the corrugated separator sheet adjacent asleeve such that crests of the corrugated separator sheet engage thesleeve; positioning the corrugated separator sheet and sleeve betweentwo wheels spaced apart a distance smaller than a height of thecorrugations; rotating the two wheels in opposite directions to feed thecorrugated separator sheet and sleeve between the two wheels, therebyforming one of the flattened portions flush with the crests.
 16. Themethod of claim 13, wherein the sleeve is substantially cylindrical. 17.The method of claim 13, wherein the non-corrugated barrier is a firstnon-corrugated barrier, the method further comprising: positioning thecorrugated separator sheet between the first non-corrugated barrier anda second non-corrugated barrier to define a first heat exchange fluidflow path on a first side of the corrugated separator sheet and a secondheat exchange fluid flow path on a second side of the corrugatedseparator sheet.
 18. The method of claim 17, wherein positioning thecorrugated separator sheet between the first non-corrugated barrier anda second non-corrugated barrier includes positioning the corrugatedseparator sheet in a substantially cylindrical space.
 19. The method ofclaim 17, further comprising: providing an inlet to the first heatexchange fluid flow path, the inlet being positioned proximate a firstend of the corrugations; and providing an outlet to the first heatexchange fluid flow path, the outlet being positioned proximate a secondend of the corrugations; wherein the acts of providing an inlet andproviding an outlet include creating apertures in one of thenon-corrugated barriers.
 20. The method of claim 19, wherein the act ofcreating apertures includes creating apertures in the non-corrugatedbarrier to which the flattened portions are secured.
 21. The method ofclaim 13, further comprising securing crests of the corrugated separatorsheet to the non-corrugated barrier.
 22. A corrugated separator sheetfor a heat exchanger, the corrugated separator sheet comprising: aplurality of metal corrugations extending parallel to one another in alongitudinal direction, the corrugations having a plurality of peaks anda plurality of troughs opposite the plurality of peaks; and a flattenedregion proximate a longitudinal end of the separator sheet; wherein theflattened region is adjacent the plurality of peaks.
 23. The corrugatedseparator sheet of claim 22, wherein the flattened region issubstantially flush with the plurality of peaks.
 24. The corrugatedseparator sheet of claim 22, wherein the corrugated separator sheet iscylindrical.
 25. The corrugated separator sheet of claim 22, wherein theflattened region is a first flattened region, and wherein thelongitudinal end is a first longitudinal end, further comprising asecond flattened region proximate a second longitudinal end of theseparator sheet, wherein the second flattened region is adjacent one ofthe plurality of peaks and the plurality of troughs.
 26. The corrugatedseparator sheet of claim 25, wherein the first flattened region issecured to a first cylindrical barrier and the plurality of troughs arepositioned adjacent a second cylindrical barrier, wherein a first flowpath is defined between the corrugated separator sheet and the firstcylindrical barrier and a second flow path is defined between thecorrugated separator sheet and the second cylindrical barrier.
 27. Thecorrugated separator sheet of claim 26, wherein the second flattenedregion is adjacent the plurality of peaks, wherein the second flattenedregion is secured to the first cylindrical barrier.
 28. The corrugatedseparator sheet of claim 26, further comprising a corrugated regionbetween the first flattened region and the second flattened region, afirst transition region connecting the first flattened region and afirst end of the corrugated region, and a second transition regionconnecting the second flattened region and a second end of thecorrugated region, wherein the first cylindrical barrier includes aninlet to the first flow path, the inlet being positioned proximate thefirst end of the corrugated region.
 29. The corrugated separator sheetof claim 22, wherein the flattened region is a first flattened regionand the longitudinal end is a first longitudinal end, the corrugatedseparator sheet further comprising: a second flattened region adjacent asecond longitudinal end of the separator sheet; a corrugated regionbetween the first flattened region and the second flattened region; afirst transition region connecting the first flattened region and afirst end of the corrugated region; and a second transition regionconnecting the second flattened region and a second end of thecorrugated region; wherein the first transition region and the firstflattened region include corrugations bent in a direction transverse tothe longitudinal direction, the corrugations overlapping adjacentcorrugations in the first flattened region.